The present invention relates to a process and an apparatus for generating peroxyacids. More particularly it relates to a process and an apparatus for generating peroxomonosulfuric acid.
It is known that peroxomonosulfuric acid (Caro's acid) is a highly reactive oxidizer able to oxidize compounds, such as phenols, complex cyanides and lignin, which cannot or only to a lesser degree be oxidized by hydrogen peroxide. However, in most commercial applications a triple salt of potassium peroxomonosulfate is utilized. The peroxomonosulfate has the advantage over peroxomonosulfuric acid in that it loses very little active oxygen during storage, and is much easier and safer to handle. On the other hand, many applications of peroxomonosulfate are limited by the high price of peroxomonosulfate owing to the neutralization and drying costs involved in the production of the salt.
Consequently, generation of Caro's acid on-site instantaneously at the moment of use has been developed (as described by E. Jourdan LaForte in U.S. Pat. No. 3,939,072). On-site generation of Caro's acid can be carried out as it is needed or in situ, and thus bypasses the disadvantage of its low storage stability. In the process as described in U.S. Pat. No. 3,939,072, concentrated H.sub.2 SO.sub.4 and concentrated H.sub.2 O.sub.2 are added simultaneously at a molar ratio which produces peroxomonosulfuric acid most economically. The economical molar ratio for that system is about 2 moles H.sub.2 SO.sub.4 : 1 mole H.sub.2 O.sub.2. The yield (i.e., the amount of active oxygen (AO) in percent which has been transferred from H.sub.2 O.sub.2 to H.sub.2 SO.sub.4 forming H.sub.2 SO.sub.5) is limited in such a process which simply mixes the reagents together at once. The maximum theoretical yield obtainable with such a conventional process is less than 75% at a molar ratio of H.sub.2 SO.sub.4 : H.sub.2 O.sub.2 being less than or equal to about 2:1.
The chemical reaction which takes place when concentrated H.sub.2 SO.sub.4 and concentrated H.sub.2 O.sub.2 are mixed together is described by the equation below: EQU H.sub.2 SO.sub.4 +H.sub.2 O.sub.2 .revreaction.H.sub.2 SO.sub.5 +H.sub.2 O
The peroxomonosulfuric acid being formed is in chemical equilibrium with H.sub.2 O.sub.2 and H.sub.2 SO.sub.4. Thus, increasing the concentration of H.sub.2 O.sub.2 and/or H.sub.2 SO.sub.4 will yield more Caro's acid (H.sub.2 SO.sub.5). On the other hand, the more water that is in the system, the more H.sub.2 SO.sub.5 is hydrolyzed back to H.sub.2 SO.sub.4 and H.sub.2 O.sub.2. Since concentrated sulfuric acid is a very strong dehydrating agent, it is more favorable to add excess H.sub.2 SO.sub.4 so that the H.sub.2 O formed is "fixed" in the hydrate of sulfuric acid. However, use of large amounts of sulfuric acid becomes uneconomical because the neutralization costs increase in excess of the increase in yield of peroxomonosulfuric acid. In addition, the higher the H.sub.2 SO.sub.4 load, the greater is the salt load in the effluent after the treatment process. As a result, the process as described in U.S. Pat. No. 3,939,072 is limited in its efficiency.
There is a long felt desire in the industry to economically increase the yield of H.sub.2 SO.sub.5, while at the same time not burdening the environment (i.e., receiving waters) with higher salt loads.
Surprisingly, we have found a process and an apparatus to produce peroxomonosulfuric acid with a yield clearly and unexpectedly exceeding that of conventional processes performed at similar concentrations of the oxyacid and hydrogen peroxide and similar molar ratios of oxyacid to hydrogen peroxide. In addition to the production of peroxomonosulfuric acid and the salts thereof, the present invention can be applied to produce other inorganic and organic peroxyacids, such as peracetic acid and performic acid, among others (e.g., perpropionic acid).